Purpose:
Loss of Mertk function causes rapid photoreceptor degeneration in rats, humans and mice. We found that a subset of Mertk knockout mice on a C57BL/6 (B6) background had areas of normal retinal structure. We hypothesized that photoreceptor degeneration in these mice is suppressed by the B6 allele of a modifier gene, which we sought to identify.

Methods:
We meiotically mapped a modifier locus, and used electroretinography (ERG) and histology to characterize the retinal phenotype of Mertk knockout mice with different modifier genotypes. We used loss-of-function, gain-of-function, and molecular and cellular biology approaches to evaluate a putative modifier gene and determine a likely mechanism for suppression of photoreceptor degeneration in Mertk-deficient animals.

Results:
Mertk-deficient animals heterozygous for B6 and 129/Ola modifier alleles exhibit an unusual intermixing of degenerating and preserved retinal regions, with more RPE phagosomes evident in preserved regions. Mertk-deficient mice homozygous for the B6 modifier allele exhibit degeneration only in the far periphery, even at 8 months of age, and have enhanced ERG amplitudes relative to those homozygous for the 129 allele. The modifier maps to an approximately 2-megabase critical interval that includes Tyro3, a paralog of Mertk. Tyro3 expression varies with modifier genotype in a manner characteristic of a cis-acting expression quantitative trait locus (eQTL), with the B6 allele conferring an approximately three-fold higher expression level. TYRO3 protein is more abundant in the RPE adjacent to preserved central retinal regions of Mertk knockout mice homozygous for the B6 modifier allele. Loss of Tyro3 function accelerates the pace of photoreceptor degeneration in Mertk knockout mice. Endogenous human TYRO3 protein co-localizes with nascent photoreceptor outer segment (POS) phagosomes in a primary RPE cell culture assay, and expression of murine Tyro3 in rat kidney fibroblasts stimulates their uptake of POS.

Conclusions:
Our findings supply definitive evidence for a significant, direct role of TYRO3 in RPE phagocytosis, demonstrate that an eQTL can modify inherited photoreceptor degeneration in mice, and suggest that eQTL may constitute a general class of modifiers of human inherited photoreceptor degenerations. Our results therefore provide a rationale for a comprehensive search for eQTL in human ocular tissues and, in particular, the RPE.